Hitherto, as a conductive filler for an anisotropically conductive adhesive, an anisotropically conductive film, and a conductive spacer, which are used in an electronic display device such as a liquid crystal display, a conductive filler has been used which is made by coating ceramic powder or resin powder with a metal such as gold, silver, copper, nickel, or the like. Regarding such conductive fillers, Patent Documents 1 and 2 have been reported. In Patent Document 1, a method of producing a conductive filler is disclosed in which fine powder of a metal is subjected to electroless plating while applying ultrasonic waves thereto. In Patent Document 2, a method of producing a silver-coated spherical phenol resin is disclosed in which a spherical phenol resin is activated by an aqueous hydrochloric acid solution of tin chloride and is then subjected to electroless silver plating. However, silver-plated films formed by such methods have problems in denseness and adhesion.
As conductive fillers that improved the problems, Patent Documents 3 to 5 have been reported. In Patent Document 3, a conductive filler is disclosed which is produced by performing nickel plating on inorganic or organic particles to form a base, then performing copper plating thereon, and further performing silver plating thereon. In Patent Document 4, a conductive filler is disclosed which is produced by performing nickel plating on base material particles to form a base, and further performing silver plating thereon. In Patent Document 5, a conductive filler is disclosed which is produced by not performing silver plating but performing nickel plating on resin powder to form a base and further performing gold plating thereon. In a current state, a conductive filler is produced by performing nickel plating on resin powder to form a base and further performing gold plating thereon, and the conductive filler is mainly used for purposes of an anisotropically conductive adhesive, an anisotropically conductive film, and a conductive spacer.
In addition, a method has been reported which includes subjecting a spherical cross-linked polystyrene resin or phenol resin to electroless plating of silver (Patent Document 6). In the electroless plating, a tin coating is formed on the surface of the spherical resin by using a solution of stannous chloride and hydrochloric acid, and then a silver coated film is formed on the tin coating.
However, in all of the production method of performing copper plating and silver plating after nickel base plating (Patent Document 3), the production method of performing silver plating after nickel base plating (Patent Document 4), the production method of performing gold plating after nickel base plating (Patent Document 5) described above, one or two layers of plating need to be further formed after performing the nickel base plating. Therefore, the number of performing plating is increased, and it takes a lot of efforts. In addition, in the case where it is used as a conductive filler for a conductive spacer, characteristics are required such as a way of crushing of a filler when a load is exerted on the filler through a weight and the rate of recovery when the weight is removed. With regard to the conductive particles of Patent Documents 3 to 5, the nickel base plating is hard; and therefore, it is required to improve the characteristics.
In addition, in the method of performing electroless plating of silver on the spherical cross-linked polystyrene resin or phenol resin (Patent Document 6), a pretreatment is performed using a solution of stannous chloride and hydrochloric acid as a tin compound, and then silver plating is performed. In this method, only a silver coated film that is inferior in denseness and adhesion may be formed on the fine particle resin made of an acryl-based resin or styrene-based resin.
On the other hand, regarding the conductive filler for an anisotropically conductive adhesive, an anisotropically conductive film, and a conductive spacer, a practical use of a conductive filler using resin fine particles made of an acryl-based resin or styrene-based resin is required from the viewpoint of a way of crushing of a filler when a load is exerted on the filler through a weight and the rate of recovery when the weight is removed.